The search box at the top right of the activity feed will help you find that document you remember.
The box is hidden by default and can be exposed by clicking the magnifying glass to the left of the yellow Data tab.
You can then type search terms in the box and hit return to search.
Search indexes several different parts of each micro-response: Document title, elaboration and the excerpt from the micro-response allowing you to search for any words contained within those elements. Each indexed element is highlighted with the orange boxes in this nugget illustration.
In the screenshot below, a search for "tests" was done within a Ponder group called "Education". By default searches are scoped to responses created "This Week" but you can use the time frame selector to expand the scope and look farther back in time.
Finally, search happens within a given Ponder group, so make sure you are looking at the activity feed you want to search when you click the search button.
You can also use search on the Public Feed.